Turnover in vivo of lactosyl-ceramide and other glycosphingolipids in the adult rat brain.

The quantitative contribution of glucose to the biosynthesis of lactosyl-ceramide and other glycosphingolipids was studied in the adult rat brain in vivo using a semicompartmental model. Half-lives of glucose carbon in both the total carbon pool and the carbohydrate residue of the lipid were calculated. In all glycolipids the half-life of carbohydrate units was six to eight times shorter than the half-life of carbon in the total carbon pool of the same lipid. This carbohydrate half-life appears to be closely related to the turnover rate of the glycolipid. The shortest carbohydrate half-life (2.2 days) was obtained for lactosyl-ceramide followed by gangliosides, galactosyl-ceramides, and sulphatides. The results indicate that lactosyl-ceramide may serve as a branch point for the biosynthesis of cerebral gangliosides in vivo, rather than occur as a breakdown product of more complex molecules.